Chemical coating process for metal



United States Patent 3,505,129 CHEMICAL COATING PROCESS FOR METAL Harold Burstein, Oak Park, and William S. Russell, Warren, Mich., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Jan. 16, 1967, Ser. No. 609,294 Int. Cl. C23f 7/26 US. Cl. 1486.2 6 Claims ABSTRACT OF THE DISCLOSURE A process for imparting a protective and/or paint base coating of the chromate type to ferrous metal surfaces. The ferrous metal surface is first given an acidic pretreatment and is then contacted with an aqueous coating solution containing hexavalent chromium, HF and a soluble cyanide, the solution having a pH of from 1.0 to 2.5. Fluoboric acid is added to the treating solution to maintain the pH within this range, Preferably, the treating solution contains at least 0.05% CrO 0.01 to 0.5% HF and 0.001 to 0.5% potassium ferricyanide and has a total fluoride content of from 0.016%

This invention relates to a process for coating metal surfaces and more particularly relates to a process for imparting a protective and/or paint base coating of the chromate type to ferrous metal surfaces.

In the past, numerous processes have been proposed and used for providing a protective and/or paint base coating of the chromate type on metal surfaces. For the most part, however, although these processes have produced satisfactory coatings on aluminum surfaces, and zinc surfaces, they have not been widely used for producing coatings on ferrous metal surfaces. Moreover, where chromating processes have been developed for coating ferrous metal surfaces, the nature of these processes has been such that they have not been easily adapted for the coating of other types of metal, such as aluminum and zinc, Similarly, the chromate coating processes which have been used satisfactorily on aluminum and zinc, have not generally been suitable for producing satisfactory coatings on ferrous metal surfaces.

In recent years, the ability of metal coating processes to coat a variety of different metals, and particularly aluminum, zinc and ferrous metals, has become of increasing importance. As more and more metal producing and metal finishing plants have begun to work with all three of these metals, it has become increasingly desirable to develop coating processes which may be satisfactorily used on aluminum, zinc and ferrous metal surfaces, with only minor modifications of the process being required when going from one metal to another.

-It is, therefore, an object of the present invention to provide an improved process for coating ferrous metal surfaces, which process may also be used for coating aluminum and zinc surfaces, with only minor modifications of the process.

Another object of the present invention is to provide an improved method for treating ferrous metal surfaces. which method is easily and economically carried out and is also adapted to the treatment of aluminum and zinc surfaces.

A further object of the present invention is to provide an improved process for treating ferrous metal surfaces to provide thereon an improved protective and/or paint base coating.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention in- 3,505,129 Patented Apr. 7, 1970 cludes a process for treating ferrous metal surfaces which comprises subjecting the ferrous surface to be coated to an acidic pretreatment, thereafter, contacting the acidtreated surface with an aqueous coating solution containing hexavalent chromium, HF and a soluble cyanide, which solution has a pH within the range of about 1 to 2.5, adding fluoroboric acid to the solution in an amount sufficient to maintain the solution pH within the above range and maintaining the surface in contact with the solution for a period sutficient to effect the formation of the desired coating on the surface. The coatings produced by this process are found to inhibit corrosion and give an increased durability to paint finishes applied to ferrous metal surfaces which have been thus-treated. Moreover, it is found that with only minor modifications of this process, it is equally applicable for producing protective and/or paint base coatings on both aluminum and zinc surfaces.

More specifically, in the practice of the present invention, a ferrous metal surfaces which is to be coated is first given an acid pretreatment. It is to be appreciated, that as used in the specification and claims, the term ferrous metal is intended to refer to surfaces of iron, steel, and the like, as well as alloys which are predominantly of iron or steel. The acid pretreatment which is given to the ferrous metal surfaces may be carried out in any convenient manner. For example, the ferrous metal surface may first be cleaned using a suitable alkaline cleaner and may then be immersed in, or sprayed wiht, an aqueous solution of an acid, such as phosphoric acid, hydrofluoric acid, sulfuric acid, nitric acid, hydrochloric acid, HBF, (fluoboric acid), or the like. Alternatively, the ferrous metal surface may be cleaned with an acidic cleaner, rather than an alkaline cleaner, whereby the acid pretreatment and activation of the metal surface is effected simultaneously with the cleaning thereof. In such instances, the need for a separate acid pretreatment of the ferrous metal surface is eliminated. Various suitable acidic cleaners can be used for this purpose, as are known to those in the art. It is to be appreciated that these include many cleaning compositions which have a pH below 7, i.e., an acid pH. Typical of these are the following:

Desirably the dry compositions, as have been set forth above, are dissolved in water to form the acidic cleaning solutions. Concentrations of these compositions within the range of about 0.5-6 ounces per gallon of cleaning solution are typical.

The wetting agent in these compositions may beanionic, cationic, nonionic or amphoteric. Additionally, these acidic cleaners may contain various other components either in addition to, or in place of those which have been specifically set forth above, as is known to those in the art.

It is to be appreciated, however, that regardless of whether the acid pretreatment of the ferrous metal surface is eifected with an aqueous acid solution in a separate treating step or simultaneously with the cleaning by using an acidic cleaner, the acidic solution used in either case desirably has a pH within the range of about 0.1 to 6.0, with pH values Within the range of about 0.1 to 5.5 being preferred. Where a separate pretreatment with an aqueous acid solution is used, the aqueous solution may contain from about .01 to by weight of the acid, depending upon the particular acid used in each instance. In using an alkaline cleaner prior to the acid pretreatment, after the ferrous metal surface has been contacted with the cleaner for a period of time sufficient to effect the desired cleaning thereof, the surface is preferably rinsed in hot water to remove any residual alkali therefrom and is then contacted with the aqueous acid solution, either by spray or immersion methods, contact times of from about 0.5 second to seconds being typical, depending upon the contacting techniques being used. Where an acidic cleaner is used, so that the acid pretreatment and cleaning are carried out simultaneously, the ferrous metal surface is contacted, preferably by immersion or by spray techniques, with the acidic cleaning solution for a period of time sufficient to effect the desired cleaning of the surface. Typical contact times when using an acidic cleaner are from about 10 seconds to 5 minutes, depending upon the contacting techniques which are used.

Following the acid pretreatment of the ferrous metal surface, whether it is effected with an acidic cleaning solution or with a separate aqueous acid solution subsequent to alkaline cleaning steps, the acid pretreated metal surface is sometimes preferably rinsed with water to remove any residual acid from the surface. Desirably, a warm water rinse is used, preferably within the range of about 20 to degrees centigrade, with typical rinsing contact times being from about 0.5 second to 2 minutes. The thus-rinsed surfaces are then contacted with an aqueous coating solution containing hexavalent chromium, HF, and a soluble cyanide, which coating solutions have a pH within the range of about 1.0 to 2.5. Desirably, the hexavalent chromium content of these coating solutions is at least about 0.05%, calculated as CrO with amounts within the range of about 0.1 to 1.3% by weight, calculated as CrO being preferred. In general, from the standpoint of the coatings produced, the maximum amount of hexavalent chromium in the solutions has not been found to be important. From the standpoint of cost however, large quantities of hexavalent chromium, e.g., in excess of about 3% by weight of the solution, calculated as CrO are generally not used.

The HP in the coating solution functions as a coating weight control agent so that the amount present will vary, depending upon the coating weight desired on the metal surface and the processing time utilized in each instance. In general, the coating solutions used will contain HF in an amount within the range of about 0.01% to about 0.5%, with amounts within the range of about 0.05% to about 0.3% of the coating solutions being preferred. It will be appreciated, however, that in some instances coating baths may be used which contain HF in amounts which are either greater than or less than the typical concentrations which have been indicated hereinabove.

The soluble cyanide component of the treating bath functions as an accelerating agent and is desirably selected from the group consisting of ferricyanic acid, ferrocyanic acid and the water soluble salts of these acids, such as the alkali metal salts and the like. By the term alkali metal it is intended to include sodium, potassium, lithium, cesium, and rubidium. Of these, the salts of sodium and potassium have been found to be particularly suitable so that primary reference hereinafter will be made to such material. In many instances, excellent results have been obtained when using potassium ferricyanide [-K Fe(CN) as the soluble cyanide compound so that hereinafter, primary reference will be made to this material. Desirably, the soluble cyanide compounds are present in the treating solution in amounts Within the range of about .001 to about .5% by Weight, calculated as potassium ferricyanide, with amounts within the range of about .005 to about 3% by weight, calculated as potassium ferricyanide being preferred.

As has been noted hereinabove, the treating solutions used in the process of the present invention have a pH within the range of about 1 to 2.5 and preferably within the range of about 1.3 to 1.9. As the metal coating bath is used, it is found that the pH of the solution rises and that as this takes place, the quality of the coating produced is adversely affected. It is, therefore, necessary to control the pH of the coating bath and maintain it within the desired range as has been indicated, so as to maintain the quality of the coating produced. This control of the pH of the operating bath is effected by adding fiuoboric acid (HBF to the operating solution in amounts sufficient to control the pH of the solution and maintain it within the desired range. Obviously, the amount of fluoboric acid added will depend upon the degree of pH reduction which is necessary and this will vary in each instance, depending upon the operating characteristics of the coating solution Which is used. In general, however, it has been found that additions of fluoboric acid to the solution in amounts within the range of about 5 to 200 grams per 1000 square feet of metal surface to be treated are typical and will maintain the solution pH within the desired ranges. In some instances, however, amounts of fluobon'c acid which are both greater than and less than those indicated as typical may also be used to obtain the desired pH control.

Generally, the coating baths of the present invention will have a total fluoride content within the range of about 0.0l-6% by weight of the bath, with a total fluoride content within the range of about 0.1-4% by weight of the bath being preferred. By total fluoride in the bath it is meant the fluoride present as HF, as well as that which has reacted to form chromium and iron fluorides and also that which is present as the fluoboric acid. In this regard, it is to be noted that in some instances it has been found to be desirable to utilize an aged bath, i.e., one containing iron and trivalent chromium. Such aged baths may contain the iron and chromium in amounts up to the saturation point of each in the solution, although maximum amounts of about 34% by Weight of each are typical. Preferably, the solutions will contain from about 0.052% by weight of each of the iron and chromium. Generally, it is found that the higher amounts will be present in strip line operations and the lower amounts in monorail operations. The reason for this is the greater drag-out encountered in monorail operations which tends to physically remove these materials, sometimes to the point that substantially no iron or chromium is present in the bath.

Although the reason is not fully understood, it has been found that in order to consistently obtain a high coating quality, the coating bath should contain both the HF as the coating weight control agent and also the fiuoboric acid as the pH control agent. Thus, where it is attempted to control pH by the addition only of HP, or to control the coating weight only by the addition of fiuoboric acid, it is not possible consistently to obtain a coating having the desired high quality. Moreover, it has further been found that nitrate, chloride, sulfate, and the silicofluoride ions are detrimental to the coating bath and the coatings produced so that the coating solutions are desirably maintained substantially free of these ions. It is to be appreciated, however, that in indicating that the coating baths are substantially free of these ions, it is not intended to exclude the presence of these ions in amounts which are sufficiently small as to havev no detrimental effect on the coating bath, as for example, amounts of these ions which may be contained as impurities in the water used in making up the coating solution.

In coating the ferrous metal surfaces in the practice of the present method, the coating solutions, as have been described hereinabove, are brought into contact with the acid pretreated ferrous metal surface to be coated. Any suitable contacting technique may be used, although spray and immersion techniques are preferred. Desirably, the treating bath is at a temperature within the range of about to about 60 degrees centigrade, with temperatures within the range of about to 45 centigrade being preferred. In general, the length of time that the ferrous surfaces are in contact with the treating solutions will depend upon the coating weights which are desired on the metal surface. Typically, the coating weights produced will be within the range of about 3 to 100 milligrams per square foot and the contact time used to achieve these coating weights will be within the range of about 0.5 second to 5 minutes. It is to be appreciated, however, that in some instances it may be desirable to produce either greater or lesser coating weights on the metal, so that in these instances, suitable contact times to produce these coating weights will be used.

After the desired coating has been produced on the ferrous metal surfaces, in many instances it has been found desirable to give the coated surfaces a final rinse with an aqueous solution containing hexavalent chromium. Such rinsing solutions and their use are well known to those in the art. Typically, the rinsing solutions used are aqueous solutions containing hexavalent chromium, calculated as CrO in amounts within the range of about 0.001 to about 0.5% by weight of the solution. Following the hexavalent chromium solution rinse, the treated surfaces are then dried and may, if desired, be given a paint coating.

In formulating the treating solutions for use in the present method, one or more concentrate compositions are formed which are then diluted with water to give a treating bath which contains the bath components in the desired amounts. In these concentrate compositions, the hexavalent chromium may be present in the form of any water soluble hexavalent chromium compound, such as chromic acid, the alkali metal or ammonium chromates and dichromates, and the like. The HF is normally present in the concentrate compositions as hydrofluoric acid, and the preferred concentrate composition, therefore, contains hydrofluoric acid and chromic acid. Generally, it has been found to be desirable if the soluble cyanide compound is not incorporated in the concentrate composition with the hexavalent chromium compound and the hydrogen fluoride, but rather is added separately to the treating bath. The reason for this is that the soluble cyanide compounds used, such as potassium ferricyanide, are somewhat unstable in acidic solutions so that in an aqueous concentrate solution, decomposition of these materials would take place before the concentrate solutions were diluted to form the treating bath.

During the operation of the treating baths in the present process, replenishment of the solutions are necessary, as is known in the art, to compensate for the depletion of the bath components due to both the chemical reaction of these components with the metal surface being treated as well as the physical removal of the bath components on the metal surface by dragout. The replenishing materials, like those used for making up the original bath solution, are typically concentrated aqueous solutions containing the desired components, in amounts and ratios which are suitable to compensate for the depletion which takes place in the bath. Here again, however, it is desirable that the soluble cyanide compounds are added separately to the treating bath rather than being combined with the other components in the replenishing concentrate material. It will, of course, be apparent that in the replenishing concentrate material, in addition to the hexavalent chromium compound such as chromic acid and hydrofluoric acid, fluoboric acid will also be present in amounts suflicient to maintain the desired control of the pH of the solution. 7

As has been' noted hereinabove, the thus-described coating baths produce an excellent protective and/or paint base coating on ferrous metal surfaces, While applied in accordance with the method of the present invention. Additionally, it has been found that where aluminum and/or zinc surfaces are also to be treated, coating of these surfaces may be effected using the same treating bath but with only minor modifications of the treating process. Where zinc and aluminum surfaces are treated, the acid pretreatment of the metal surface, prior to contact with the coating solution, has generally not been found to be necessary. Additionally, the pH control of the operating bath by the addition of fluoboric acid, has likewise \generally not been found to be necessary for the treatment of aluminum and zinc. Accordingly, by eliminating the acid pretreatment and by replenishing the bath only with chromic acid, hydrofluoric acid, and the potassium ferricyanide, aluminum and zinc surfaces may also be treated to obtain thereon a protective and/or paint base coating.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, parts and percents are by weight and temperatures are in degrees centigrade. It will be appreciated, however, that these examples are merely exemplary of the present invention and are not to be taken as a limitation thereof.

EXAMPLE 1 Steel panels were cleaned by spraying for 30 seconds with an alkaline cleaning solution at a temperature of about 72 degrees centigrade. Thereafter, the panels were rinsed by spraying for 30 seconds with hot water and were then given an acid pretreatment by being immersed for 10 seconds in a 10% by volume aqueous solution of sulfuric acid. The acid treated panels were then again rinsed, by spraying with water for about 30 seconds, after which they were sprayed with a chromate coating solution for 20 seconds at a temperature of about 40 degrees centigrade. The chromate coating solution used had a pH of about 1.6 and contained the following components in the amounts indicated:

Components: Grams per liter CIO3 5 HF 0.7 Potassium ferricyanide 0.3

Thereafter, the thus-coated panels were then rinsed by spraying for 10 seconds with cold water and then dried. In each instance, an excellent adherent coating was preduced on the panels thus-treated.

EXAMPLE 2 The process of Example 1 was continued and additional steel panels were processed, the overspray of the chromate coating solution being continuously collected and recycled. As additional panels were thus-treated, the pH of the chromate coating solution gradually increased to about 1.9. At this point, 3 grams per liter of fluoboric acid were added to the coating solution to bring the pH down to the desired level of about 1.6. Thereafter, additional quantities of fluoboric acid were added to the coating solution to maintain the pH at this level, and there was also added chromic acid, hydrofluoric acid, and potassium ferricyanide to maintain the concentration of these materials in the coating solution at substantially that as originally formulated. As in the previous example, an excellent, adherent coating was produced on the panels thus treated.

EXAMPLE 3 The procedure of Example 2 was repeated with the exception that no hydrofluoric acid was added to replenish the chromate coating solution, the replenishing material added being only chromic acid, potassium ferricyanide, and fluoboric acid. Although the panels initially treated had an excellent coating produced thereon, there was a gradual inhibition of the coating formed, although the pH of the coating solution was maintained at about 1.6,

until the point was reached at which no coating was formed on the panels treated.

EXAMPLE 4 The procedure of Example 2 was repeated with the exception that no fiuoboric acid was added as a replenishing material to maintain the pH of the coating solution at about 1.6. In this instance, the replenishing materials added were chromic acid, hydrofluoric acid and potassium ferricyanide. Although the panels initially treated had an excellent coating produced thereon, there was a gradual inhibition and deterioration of the coating formed until a pH of 2.7 was reached, at which point no coating was formed on the panel.

EXAMPLE 5 EXAMPLE 6 The procedure of Example 2 was repeated with the exception that the acidic pretreatment was effected using an acidic cleaning solution, rather than an alkaline cleaner and an acid rinse. The cleaner used was formulated by dissolving a dry composition of 90% by weight of monosodium phosphate and 10% by weight of a nonionic wetting agent, in water in an amount of 2 ounces per gallon. The steel panels were cleaned with this solution by spraying for 30 seconds at about 72 Centigrade, the solution having a pH of about 5.2-5.4. The panels were then rinsed by spraying with hot water for 30 seconds and were then coated by spraying for seconds using the coating solution and method of Example 2. As in this example, an excellent adherent coating was produced on the panels treated.

This acid cleaning solution was also used in the processes of Examples 3-5, in place of the alkaline cleaner and the acidic rinse. In each instance results similar to those in these examples were obtained.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are possible and it is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing the same result in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A process for coating ferrous metal surfaces which comprises subjecting the ferrous metal surface to be coated to an acidic pretreatment, thereafter contacting the acid treated surface with an aqueous coating solution containing hexavalent chromium, calculated as CrO in an amount of at least about 0.05%, HP in an amount within the range of about 0.01 to 0.5%, and a soluble cyanide, calculated as potassium ferricyanide, in amount within the range of about 0.001 to 0.5%, which solution has a total fluoride content within the range of about 0.001 to 6% and a pH within the range of about 1 to 2.5, adding fiuoroboric acid to the solution in an amount sufllcient to maintain the solution pH within the range of 1 to 2.5 and maintaining the ferrous metal surface in contact with the solution for a period sutficient to eifect the formation of the desired coating thereon.

2. The process as claimed in claim 1 wherein the pH is within the range of about 1.3 to 1.9.

3. The process as claimed in claim 2 wherein the terrous metal surface is first cleaned with an alkaline cleaner and then contacted with an aqueous solution of an acid to elfect the acid pretreatment.

4. The method as claimed in claim 2 wherein cleaning and the acid pretreatment of the ferrous metal surface is effected simultaneously by contacting the metal surface with an aqueous acidic cleaning composition.

5. The method as claimed in claim 4 wherein the coating solution contains hexavalent chromium, calculated as CrO in an amount within the range of about 0.1 to 1.3%, HF in an amount within the range of about 0.05 to 0.3%, a ferricyanide, calculated as potassium ferricyanide, in an amount within the range of about 0.005 to 0.3 and a total fluoride content within the range of about 0.1 to 4%.

6. The method as claimed in claim 5 wherein the fiuoboric acid is added to the treating solution in an amount within the range of about 5 to 200 grams per 1000 square feet of metal surface treated.

References Cited UNITED STATES PATENTS 2,114,151 4/1938 Romig 148-62 2,357,219 8/1944 Mott 1486.2 X 2,796,370 6/1957 Ostander et a1 1486.2

FOREIGN PATENTS 714,541 9/ 1954 Great Britain. 990,352 4/ 1965 Great Britain.

RALYl-I S. KENDALL, Primary Examiner US. Cl. X.R. 148-627 

